1. Field of the Invention
The present invention relates to a lean burn internal combustion engine.
2. Description of Related Art
To improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing ratio of specific heats. Flatly speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burning. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomized fuel or vaporized fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition has been proposed for operating an engine with very lean air/fuel mixtures.
When certain conditions are met within a homogeneous charge of lean air/fuel mixture during low load operation, auto-ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high thermal efficiency. NOx emission produced in controlled auto-ignition combustion is extremely low in comparison with spark ignition combustion based on propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by spark ignition engine and diesel engine, respectively, the burnt gas temperature is highly heterogeneous within the charge with very high local temperature values creating high NOx emission. By contrast, in controlled auto-ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled auto-ignition combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote auto-ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, auto-ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio.
In all the above cases, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow. The fuel used also has a significant effect on the operating range, for example, diesel fuel and methanol fuel have wider auto-ignition ranges than gasoline fuel.
An auto-ignition, which is induced by heating fuel and significantly increasing the compression ratio, in four-stroke gasoline engine is described in U.S. Pat. No. 5,535,716, which claims priority of Japanese patent application No. 6-150487 that was laid open as JP-A 7-332141 on Dec. 22, 1995. Gasoline fuel is injected inside the intake port a considerable amount of time before the intake valve is open so that the mixture of air and gasoline in the intake port is sufficiently heated before entering the combustion chamber. The mixture is ignited by compression ignition performed at high pressure. Since the gasoline fuel injected in the intake port is completely evaporated before entering the combustion chamber, reliable compression ignition is achieved. The compression ratio ranges from about 14 to about 20. Use of a compression ratio of 17.7 is described as the most preferred implementation in this publication.
Injection of the gasoline fuel is performed during a lo predetermined period from 10 degrees of crankshaft angle before the intake valve is closed to 110 degrees of crankshaft angle before the intake valve is opened.
JP-A 10-266878 discloses a technique to accomplish auto-ignition of gasoline fuel over a predetermined load range from light load to high load by adjusting closing timing of an exhaust valve as well as opening and closing timings of an intake valve. According to this known technique, over the predetermined load range, an opening timing of the exhaust valve is held invariable at a crank position before a bottom dead center (BDC) position of expansion stroke against variations of load request, but a closing timing of the exhaust valve is adjusted to varying crank positions before a top dead center (TDC) position of exhaust stroke against varying load request. A throttle valve is fully opened over this predetermined load range. The closing timing of the exhaust valve advances as load request becomes low to increase the amount of exhaust gas remaining in a cylinder to decrease the amount of fresh charge. In order to retain exhaust gas, the opening timing of the intake valve is adjusted to varying crank positions after the TbC position of exhaust stroke against varying load request over the predetermined load range. The opening timing of the intake valve retards as load request becomes low. The closing timing of the intake valve is adjusted to varying crank positions after a bottom dead center (BDC) position of intake stroke against varying load request over the predetermined load range. The closing timing of the intake valve retards gradually as load request becomes high over a portion of the predetermined load range and then advances gradually as load request becomes high over the remaining portion of the predetermined load range. According to this known technique, closing timing of the exhaust valve controls the amount of exhaust gas in the cylinder, thereby controlling available combustion chamber volume for receiving fuel mixture thereby controlling load. Further, closing timing of the intake valve advances during operation with load where auto-ignition is difficult to accomplish. Advancing closing timing of intake valve increases a compression ratio thereby increasing temperature of the mixture in the cylinder.
U.S. Pat. No. 6,135,088, which corresponds to JP-A 11-182246, shows a controlled auto-ignition engine operating process. According to this process, exhaust gas recirculated from an exhaust port through an EGR pipe is admitted to the combustion chamber, and thereafter a mixture of air and fuel is admitted through an intake port into the combustion chamber at a start of compression phase of the combustion chamber. Admission of the air fuel mixture produces stratification between the mixture and the exhaust gas to provide favorable conditions for auto-ignition within the combustion chamber.
According to this known process, the exhaust gas is used to elevate the temperature of the air and fuel mixture admitted into the combustion chamber to assist in establishing favorable condition for auto-ignition around top dead center of piston compression stroke.
An object of the present invention is to widen auto-ignition range of a lean burn internal combustion engine.
In one aspect of the present invention, the above object is achieved by a lean burn internal combustion engine having at least one cylinder with a piston reciprocating therein to define a combustion chamber. The engine comprises:
a first device for supplying first oxygen containing gas to the combustion chamber;
a second device for supplying second oxygen containing gas to the combustion chamber for producing stratification, within the combustion chamber, of first gas content with second gas content, the first gas being higher in temperature than the second gas; and
a fuel injection system for carrying out a first injection of gasoline fuel into the combustion chamber for dispersion within the second gas content, and for carrying out a second injection of gasoline fuel into the combustion chamber for dispersion within the first gas content, thereby to accomplish auto-ignition of gasoline fuel within the first gas content of the combustion chamber.